The present invention relates to a technique for processing defects in the sectors of a magnetic disk, and more particularly, to a sector defect processing technique suitable for ensuring the reliability of a magnetic disk drive which operates at low temperature.
Magnetic disks for use in magnetic disk drives are each manufactured by stacking magnetic layers on a substrate using a thin-film forming technique such as sputtering. Even a slight depression or projection on the surface of the glass substrate itself, changes in conditions during the sputtering process, or imperfect surface treatment may cause the magnetic layer surface defects that the magnetic disk, as a recording medium, does not perform its intended function properly.
The surface defects here focus attention on sector-by-sector data reading/writing reliability and do not refer only to defects serious enough to immediately cause any trouble relating to data reading from/writing into the magnetic disk drive. They also include such latent defects that cause a reading error to easily occur in comparison with other almost sectors if various conditions overlap. An error correction code (ECC) for detecting and correcting any disparities in bit strings between the data written on the magnetic disk and the data read out therefrom, is provided in the magnetic disk drive. For practical use, therefore, there is no problem if the number of bits in which a reversal error is occurring due to very small defects on the magnetic disk is within the error correction capability of ECC.
If any sectors with a latent defect are left intact, however, an increase in physical defect size is likely to increase the number of reversal bits too significantly to be correctible with ECC. Exposure of the magnetic disk to a high-temperature environment for an extended time, in particular, accelerates the expansion of the defect. At the manufacturing stage of magnetic disk drives, therefore, before the disk drives are shipped, the appropriate measure must be taken so that even a minute defect is detected with high sensitivity to prevent the defective section from being used. Among the methods of detecting defective sectors on a magnetic disk is a read/write testing method in which test data is written onto the mounted magnetic disk of the magnetic disk drive and then read out therefrom to confirm whether the written data can be read out properly.
With the increased use of magnetic disk drives for diverse purposes in recent years, opportunities for magnetic disk drive usage at low operating ambient temperatures near 0° C. are increasing. For example, HDD recorders are operated in automatic recording mode late in the night of a low room temperature or the images that were acquired using video cameras are recorded outdoors. At lower operating environmental temperatures, magnetic disk drives tend to decrease in data-reading performance since thermal shrinkage of each of the respective heads increases the interval between the magnetic disk surface and the head. During read/write tests, therefore, even a defective sector not causing the defect to emerge until the disk drive has been placed in first use at a low operating environmental temperature requires pre-shipping detection and appropriate correction.
Patent Document 1 (Japanese Patent Laid-open No. 2004-5850) discloses a technique for detecting defects in the recording medium of a magnetic disk drive very accurately and with ease. The invention described in this document uses a Viterbi decoder to process the data read out from the sectors on the recording medium, compares the data with reference data, and detects Viterbi errors. After this, the invention examines each sector and judges the presence or absence of defects from the length of the data that is causing the succession of Viterbi errors. Patent Document 2 (U.S. Pat. No. 6,088,176) discloses a technique in which the signals that have been read from a magnetic disk by using FIR are first separated into magnetic signals and thermal signals and then the thermal signals are used for defect detection of the disk.